Triode detector for radar



April 19, 1955 G. WILKES TRIODE DETECTOR FOR RADAR 2 sneaks-sheet 1 Filed Sept. 20. 1951 INVENTOR. GILBERT WILKES irulilllllll'iaiplnlllcm lllllllllill A/I or-ng April 19, 1955 G. WILKES 2,706,797

TRIODE DETECTOR FOR RADAR 7 Filed Sept. 20, 1951 2 Sheets-Sheet 2 "TANK" cmcun' TUNED TO I GRID-BIAS SOURCE INVENTOR. GILBERT- WILKES United States Patent TRIODE DETECTOR FOR RADAR Gilbert Wilkes, Chevy Chase, Md., assignor to the United States of America as represented by the Secretary of the Navy Application September 20, 1951, Serial No. 247,533

6 Claims. (Cl. 315-39) The present invention relates in general to detectors and oscillators of the thermionic type for use at microwave frequencies, and more specifically to triodes suitable for connection to coaxial cable.

Heretofore, while tubes suitable for use as detectors at microwave frequencies were known, they were not capable of acting also as local oscillators. Moreover, they coupled poorly into waveguides or coaxial cables, with resultant poor utilization of the very minute energy available in radar echo signals, and were so devoid of rigidity that their microphonic effects far overshadowed the said signals, thus making these tubes relatively inefficient for their intended use.

It is therefore an object of the present invention to provide a thermionic tube having such characteristics that it may act as an efficient detector at microwave frequencies.

Another object is to provide a thermionic tube for use at microwave frequencies, that is free from microphonics, even under conditions of severe vibration.

A further object is to provide a thermionic tube suitable for use as a local oscillator at microwave frequenc1es.

7 An additional object is to provide a thermionic tube of this kind that is composed entirely of metal except for minor insulation elements, and that may therefore be manufactured to very closely held dimensions to ensure uniformity.

A particular object is to provide a thermionic tube of this kind that has a high Q when used at microwave frequencies, so that it may utilize the signal energy with great eificiency and may also oscillate efliciently.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. 1 is an axial section, partly in side elevation, a triode embodying the invention;

Fig. 2 is an enlarged detail cross section through the central portion of the triode including the filament, filament-support and grid, the section being made in the plane 22 of Fig. 1;

Fig. 3 is a side elevation of the triode, showing also a diagram of a circuit that may be used therewith;

Fig. 4 is the corresponding end view; and

Fig. 5 is a perspective view of the filament support and filament.

Referring first to Fig. 1, there is shown a triode having a filamentary cathode 11 carried by an insulating heat-resistant support or core 12 preferably made of a ceramic material such as porcelain.

The cathode 11 is surrounded by a grid 13, best shown in Fig. 2, consisting of the mid-portion of a cylindrical conducting tube 14, which is slotted to provide a plurality of radially located grid bars 15, with slots 16 therebetween. The non-slotted end portions of this tube 14 thus form rigid supports for the grid 13, integral with said grid. Beyond the grid 13 and located relatively far therefrom in a radial direction, is the anode 17 which is in the form of an annulus or ring. These are the chief electrical elements of the triode, but certain other features are also of importance in properly supporting them, for providing a vacuum-tight chamber, high-frequency chokes, and resonant cavities and other requisite features which are described fully below.

The refractory core 12, Figs. 1, 2 and 5, consists in the present instance of a central disk 18 having on each face thereof conical frusta 19 and 20, in axial alinement with said disk, the whole being formed as a single rigid piece of porcelain or equivalent heat-resistant dielectric material. Holes 21 and 22 may be made through the core 12 in molding the biscuit of unbaked porcelain clay. The holes 22 are located near the periphery of the central or disk-shaped portion 18 and extend parallel to its axis, thus providing channels through which passes the wire that forms the cathode 11. After this wire is in place in the holes 22 with its free ends extending out through the holes 21 in the frusto-conical end portion 19, for convenience in later making the electrical connections thereto, all the remaining spaces in the holes or channels in the core are filled completely with slip, or other glaze or thermo-setting material, and thereupon the ceramic core is fired or burnt in the conventional way to form a rigid mass in which the wire is embedded firmly.

it will be understood that the holes or channels in which the cathode 11 is mounted are preferably as close to the periphery of disk 13 as feasible in the manufacture of the device. These holes should also be equidistant from the axis of the core 12, as the peripheral portion of the disk 18 is later ground away to provide a true cylindrical surface of accurate radial dimension, with the successive segments of wire 11 exposed in said surface to act as an electron-emitting cathode when hot. Accuracy in placing the wire 11 is highly important in that it makes close tolerance in the diameter of the disk possible, which in turn is important in that it makes possible the highly desirable very close spacing between the cathode 11 and the grid 13 that surrounds it.

The core 12 is conveniently supported in a rigid way by having its frusto-conical end portions 19 and 20 received in similarly shaped metal cups. One of these cups is shown at 23 in Fig. 1. It has an outer end portion 24 fitting within the outer solid end portion of the slotted tube 14 and silver-soldered into said tube as shown at 25 to provide a vacuum-tight seal that is electrically conducting and mechanically rigid. A hollow 23a in the inner end of cup 23 is designed to allow said cup to fit closely about the frusto-conical end portion 19 of the core 12. An annular slot 26 is provided around cup 23, as shown, by making a part of said cup of reduced outer diameter. This slot is properly proportioned to act as a microwave choke for the wavelength in use.

At the other end of the core 12, the terminal 27 of a central conductor 28 of a coaxial cable 29 is connected to the triode 10 by a conventional fitting 39. Terminal 27 has a hollow 34 therein that is shaped to receive and fit closely over the frusto-conical end portion 20 of the core, as shown, to support said portion 20 and hold it rigidly in place. The terminal 27 in turn is supported rigidlyby a glass seal 32 which insulates the tube 14 from an outer tubular conductor 31 of the cable 29 and also forms a vacuum-tight joint between terminal 27 and tube 14. A dummy" glass seal 33 fitting tightly in the tube 31 and spaced one-half wave length from the seal 32 is provided to secure electrical matching, and to stiffen the structure mechanically against vibration. To provide proper coupling between terminal 27 and the cathode 11, the depth of the hollow 34 that receives the end 20 of the porcelain core 12 should be equal to the wave length in use divided by twice the index of refraction of the core material, thus forming a half-wave choke. A choke 43 surrounds the seal 32. This is a half-wave choke, and provides a high-frequency path between grid tube 14 and outer conductor 31, around the insulating seal 32.

An annular resonant cavity 35 is formed around the grid tube 14 by two radial flanges 36 and 37 and a cylindrical wall 38. However, it is necessary to interrupt this cylindrical wall 38 at two places, to accommodate the anode ring 17 and the two circumferential glass beads 39 that hold said ring in place and insulate it electrically from the wall 38. To eliminate the effect of this interruption of electrical continuity of the wall 38, an annular choke 44) of radial extent equal to onehalf the wave length is provided in the position shown,

straddling the anode 17 and thus concealing this discontinuity from the resonant cavity 35.

The use of the half-wave chokes 40, 43 and 34 Will be well understood by those familiar with radio-frequency plumbing techniques, as necessary to establish a continuous wave transmission path where a metallic discontinuity is required for other reasons.

In order to permit evacuation of the triode, a hole 41 is provided through the anode and a pump connection 42 is provided on the outer cylindrical Wall of the choke 40. To make the triode vacuum-tight, the joints of cavity 35 and of choke 40 must all be tight, as must also the joints at the ends of tube 14. The filament leads 11a and 11b pass through insulators 11d in cup 23 and through a glass seal 110. The anode terminal 44 is led out through an insulating vacuum seal 45 in a wall of the choke 40 as shown. The grid connection 46 may be tube but electrically insulated therefrom, said inner condtictor having a portion thereof to mate with said projection on said core to assist in supporting said core and also to provide electrical coupling between said cathode and the central conductor of said coaxial cable.

3. In a thermionic device for connecting directly to a coaxial cable having a' central conductor and an outer tubular conductor said device having an anode, a grid and a heatable cathode, a core of refractory material, said heatable cathode being carried by said core, means providing transition between said thermionic device and coaxial cable, said means including an element for supporting said core and also for providing electrical cousecured to any convenient point of the grid assembly, for y instance, as indicated in Fig. 1.

While the specific circuit connected to the triode does not form a part of the present invention, as the triode may be utilized in a large variety of circuits, nevertheless one example of an appropriate circuit is shown in Fig. 3, purely in an illustrative sense. This circuit, wherein the triode acts as a detector and local oscillator, may include a tank circuit comprising an inductance 48 connected in parallel with a capacitor 47, the tank circuit being resonant at the desired intermediate fre quency. The tank circuit is interposed in series in the anode circuit, which comprises anode lead 44, conductor 49 and anode resistor 50, through which connection is made to the +B terminal of a power source. A connection is made to the grid 13 through conductor 46 and through a grid bias source 53. If grid bias source 53 is designed to operate as an automaticgain control means, the net signal detected by the tube is obtained from the grid potential at conductor 46. Otherwise the detected signal appears on conductor 52.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. it is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A thermionic device including a refractory dielectric core, said core centrally located therein having oppositely located projections, supports engaging said projections to secure said core in operative position, a cathode mounted on said core, a grid surrounding said cathode, inner and outer walls defining a resonant chamber surrounding said grid, the inner wall of said chamber including said grid, an anode surrounding said grid and located in the outer wall of said chamber but electrically insulated therefrom, and an annular choke surrounding the resonant chamber and extending laterally of both sides ofsaid anode.

2. In a thermionic device for connecting to a coaxial cable having a central conductor and an outer tubular conductor, said device having an anode, a grid, and a heatable cathode, a core of refractory material having a projection located thereon, said cathode being mounted on said core, means providing transition from said coaxial cable to said thermionic device, said transition means including an outer conducting tube and an inner conductor rigidly supported by said outer conducting pling between said cathode-and central conductor of said coaxial cable.

4. In a thermionic device for connecting directly to a coaxial cable and having a housing including therein an anode, a grid, a heatable cathode, a core of refractory material, said core having a central cylindrical portion with coaxial tapered lugs at both ends thereof, said cathode being carried by said cylindrical portion, and means cooperating with said coaxial cable and one of the lugs of said core for partially supporting said core and also for providing an electrical coupling between said cathode and central conductor of said cable, and a conducting cup member located on the opposite side of said core from said last mentioned means and shaped to receive one of said tapered lugs to partially support said core.

5. In a thermionic device for connecting directly to a coaxial cable and having an anode, a cathode, a grid, an insulating support for said cathode, means engaging said insulating support to hold said cathode rigidly in place and also for providing an electrical coupling between said cathode and the central conductor of a coaxial cable, and means rigidly holding said grid in spaced relationship to said cathode, said means including a metallic support connected to said grid, said engaging means and said holding means having a slot formed therebetween, said holding means being coupled to said cathode but isolated from said cathode by said slot acting as a choke.

6. in a thermionic device for connecting directly to a coaxial cable having a central conductor and an outer conducting tube, said device including a cathode, an anode, a refractory core having said cathode mounted thereon, conducting means associated with said core and central conductor of said coaxial cable, said means supporting said core and providing an electrical coupling between said central conductor of said coaxial cable and said cathode carried by said core, a conducting member surrounding said cathode and including a grid, means insulating said conducting member from said outer conducting tube of said coaxial cable, and a half-wave choke adjacent said insulating means and providing a high-frequency path between said conducting member and said outer conducting tube of said coaxial cable.

References Cited in the file of this patent UNITED STATES PATENTS 

